The development of murine monoclonal antibodies and their proteolytic Fab fragments has raised interest in their utility as diagnostic and therapeutic reagents for in vivo imaging and drug targeting. However, successful in vivo targeting of radionuclides, drugs or toxins using 150 kD intact antibodies or their 50 kD Fab fragments (an antibody fragment consisting of one light chain and approximately half of the heavy chain held together by a single disulfide bond) have been restricted by the limited penetration of these molecules from the vasculature into the tissues of interest, and by their slow clearance rates in vivo, which for IgG leads to behavior that requires several days to clear the background enough for imaging to be possible. Other disadvantages of the intact antibodies or their Fab fragments include: their immunogenicity when prepared from different species, their nonspecific binding to many normal tissues and organs, and the fact that they contain multiple proteolytic cleavage sites which result in their degradation during their circulation in vivo.
Although Fv fragments, which consist of one V.sub.H and one V.sub.L domain held together by noncovalent interactions, form the minimal region of an antibody that contains a complete antigen combining site, dissociation of the V.sub.H and V.sub.L domains in vivo can preclude their use as therapeutic or imaging agents. Although Moore et al., (U.S. Pat. No. 4,642,334) and Glockshuber et al., (1990, Biochem. 29, 1362-1367) disclose attempts to stabilize these Fv fragments with engineered intermolecular disulfide bonds, monovalent 50 kD Fab and Fab' fragments have, until recently, been the smallest proteins available for effective immunotargeting.
Recently, single-chain Fv (sFv) polypeptide chains of about 27 kD have been developed containing covalently linked V.sub.H -V.sub.L polypeptides. The V.sub.H - and V.sub.L -domains are connected by a polypeptide linker. The resulting sFv polypeptide chains are also referred to in the art as biosynthetic antibody binding sites or BABS and preferably are encoded by a single DNA sequence. For a detailed description of these biosynthetic polypeptide chains see for example, Huston et al., 1988, Proc. Nat. Aca. Sci. USA 85:5879-5883 or U.S. Pat. Nos. 5,091,513 and 5,132,405, all of which are hereby incorporated by reference. The sFv polypeptide chains provide attractive alternatives to intact immunoglobulins and Fab fragments due to their small size and their stability at concentrations that typically promote dissociation of natural Fv fragments. U.S. Pat. Nos. 5,091,513 and 5,132,405; Huston et al., ((1991) Methods in Enzymology 203: 46-88; Huston et al (1993) Int. Rev. Immunol. 10: 195-217) disclose the utility of sFv polypeptides, as well as single chain constructs synthesized from single DNA sequences, which may further comprise ancillary effector proteins, such as a second sFv or a cytotoxic agent.
Pack et al. ((1992) Biochem 31: 1579-1584) disclose the construction of "mini-antibodies". The mini-antibodies are sFv polypeptide chains which also include an "oligomerization domain" at their C-termini, separated from the sFv by a hinge region. The oligomerization domains comprise self-associating .alpha.-helices, for example, leucine zippers, that can be further stabilized by additional disulfide bonds. The domains are designed to be compatible with vectorial folding across a membrane, a process thought to facilitate in vivo folding of the polypeptide into a functional binding protein.
PCT application PCT/US92/09965, published Jun. 10, 1993 also discloses the construction of bivalent sFv constructs, including crosslinked dimers. However, the pharmacokinetic properties of these constructs or those disclosed by Pack et al. are not measured in vivo.
PCT application PCT/US92/07986, published Apr. 1, 1993 discloses bifunctional (Fab').sub.2 molecules composed of two Fab' monomers linked through cysteine amino acids located at the C-terminus of the first constant domain of each heavy chain. PCT application PCT/US92/10140, published Jun. 10, 1993 also discloses bifunctional (Fab').sub.2 dimers which, in addition to the cysteine residues located in the hinge region, also contain C-terminal leucine zipper domains that further stabilize the (Fab').sub.2 dimers. In both cases, the resulting (Fab').sub.2 dimers (&gt;100 kD in size), although smaller than intact immunoglobulins, are significantly larger than sFv polypeptides and are anticipated to have slower tissue biodistribution and clearance rates following in vivo administration.
Cumber et al. disclose the generation of (Fv-Cys).sub.2 heterodimers by chemically crosslinking two V.sub.H -cys domains together (Cumber et al., 1992, J. Immunology 149B: 120-126). Although the crosslinked V.sub.H chains appear to be stable, dissociation of the V.sub.L polypeptides from each Fv reduces the pharmacological value of these constructs in vivo.
It is an object of the instant invention to provide biosynthetic constructs having enhanced pharmacokinetic properties as in vivo targeting agents. In particular, it is an object of this invention to provide biocompatible constructs having accelerated in vivo biodistribution and body clearance rates than that of antibodies or antibody fragments. It is another object of the invention to provide biosynthetic constructs having enhanced avidity in vivo, including enhanced target tissue specificity and target tissue retention. Yet another object is to provide dimeric biosynthetic constructs having improved tissue imaging and drug targeting properties in vivo. Still another object is to provide diagnostic and therapeutic formulations comprising these constructs, having particular utility in the diagnosis and treatment of malignancies. Still another object is to provide constructs having enhanced pharmacokinetic properties as in vivo targeting agents, particularly as in vivo imaging agents, for ovarian and breast tumor tissue.
These and other objects and features of the invention will be apparent from the description, figures and claims which follow.